Virtually all multicolor electrophotographic systems presently in use require a toning station that has the capability of toning or not toning an electrostatic image passing it. Most commonly, a series of electrostatic images are formed on an image member, and those images are moved past a series of toning stations, each toning station containing a different color toner. Each station applies toner to one of the images but not the others, creating a series of different color toner images on the image member. The toner images are generally transferred in registration to a receiving sheet or other receiving surface to form the multicolor image.
Another type of apparatus involves forming a first electrostatic image on a single frame and toning that electrostatic image. With the first image in place, a second electrostatic image is formed on the same frame, and that image is toned with a toner of a different color. This method could be carried out with an apparatus in which a toning station does not have to be turned off when an electrostatic image is passing it. However, its most common commercial embodiment uses a single electrostatic image-forming means and a rotary drum image member. With such structure, an electrostatic image can pass several toning stations before it is toned.
In each of these commercial apparatus, it is necessary that a toning station be capable of toning one electrostatic image and not toning another electrostatic image passing it. This capability of toning stations has conventionally been accomplished by moving the toning station away from the image member when it was not to tone the electrostatic image passing. However, to eliminate the necessity of power consuming and complicated moving mechanisms, other approaches have been suggested.
U.S. Pat. No. 4,671,207 issued to T. K. Hilbert Jun. 9, 1987, shows a magnetic brush in which developer is transported from a sump area to an applicator by a fluted roller. Developer is attracted to the fluted roller by a magnet inside the roller. The applicator has a rotatable magnetic core within an also rotatable non-magnetic sleeve. A developer valve is positioned between the fluted roller transport and the applicator to permit turning the toning station off without moving the toning station away from an electrostatic image carrying image member. This valve or gating structure enables the toning station to not tone some electrostatic images passing it without the need for moving the entire station away from its development position. Unfortunately, developer attracted by the applicator has a tendency to clog at the gate. The gate itself can clog and not pass developer.
U.S. Pat. No. 4,690,096 granted Sep. 1, 1987 to Hacknauer et al, shows a toning station similar to that in the Hilbert patent in which the gating structure has been changed to a movable shell around and spaced from the fluted roller which shell has several openings for developer. Movement of the shell can turn the toning station to an "off" or non-toning condition. For other related structure, see U.S. Pat. No. 4,748,471, Adkins, issued May 31, 1988; and U.S. Pat. No. 4,956,675, Joseph, issued Sep. 11, 1990. This structure requires the extra structure of a rotatable tube and location and size of the openings in the tube are critical, especially the opening facing the applicator.
See also, U.S. Pat. No. 4,956,674, Kalyandurg, issued Sep. 11, 1990; U.S. Pat. No. 4,671,207; U.S. Pat. No. 4,699,495; Japanese Kokai 60-194476, published Oct. 2, 1975; and Xerox Disclosure Journal, "Rapid Cutoff Development for Highlight Color", P. F. Morgan, Vol. 12, No. 2, March/April 1987.